1. Field of the Invention
The invention concerns a method for regulating the transmission ratio of a power-branched automatic transmission. The invention furthermore concerns a power-branched automatic transmission for carrying out the method of the invention.
2. Description of the Related Art
Transmissions with a continuously variable transmission ratio (CVT transmissions) are being increasingly installed in passenger cars because of their higher comfort and the reduced fuel consumption that is possible relative to automatic transmissions operating in steps and exclusively with planetary gear sets. Power transfer takes place in CVT transmissions for example through an endless torque-transmitting means that circulates between two pairs of conical disks, whereby the effective radius of each conical disk pair is variable by changing the spacing between the disks. Other steplessly adjustable transmissions are based upon rolling elements that run frictionally engaged between suitable toroidal surfaces or other principles. CVT transmissions with the greatest possible spread (ratio of the maximum transmission ratio to the minimum transmission ratio) are advantageous. At this time, values of up to >6 are being achieved. A constant contact pressure between the two bodies situated in frictional engagement with each other is necessary to produce a frictionally tight structure. With belt-driven conical pulley transmissions, a contact pressure dependent upon the torque transferred at that moment is generally produced by contact pistons. That torque contact pressure is superimposed by an adjustment contact pressure, by means of which the transmission ratio adjustment takes place. An adjustable difference of the contact pressure acting on the pairs of conical disks is needed for adjusting the transmission ratio.
For reasons of reduced fuel consumption, it is desirable to enlarge the spread of a transmission above the value that is possible with a single CVT transmission. That is accomplished with power-branched transmissions in which the spread or the transmission ratio range of the CVT transmission utilized is “doubled” in that by a combination with a gear transmission, and activation of corresponding control clutches, the transmission ratio range of the CVT transmission is run through twice in the opposite direction when changing the overall transmission ratio of the power-branched transmission over its entire spread range.
FIG. 1 shows a basic structure of a motor vehicle power train with a power-branched transmission.
A drive engine of a motor vehicle, for example, an internal combustion engine 2, is connected through a starting clutch 4 with an input shaft 6 of a power-branched transmission 8, whose output shaft is designated by 10.
The power-branched transmission 8 contains a variator 12, or variable speed drive, in the form of a continuously variable transmission, and at least one gear transmission 14 as well as at least two control clutches K1 and K2, with which the variator 12 can be coupled in various ways with the gear transmission 14. Inputs of an electronic control or regulating apparatus 16 are connected with an accelerator pedal sensor 18, a power actuator position sensor 20 of the internal combustion engine 2, an engine rotational speed sensor 22, a sensor 24 for an input shaft of the variator 12 that can be the input shaft 6 at the same time, a sensor 26 for detecting the rotational speed of the output shaft of the variator 12, a sensor 28 for detecting the rotational speed of the output shaft 10 as well as if necessary further sensors. Output signals are generated in the electronic control or regulating apparatus as a function of the signals of the sensors and algorithms, characteristic fields, etc. stored in the control apparatus 16, with which are controlled a power actuator 30 of the internal combustion engine 2, an actuator for the starting clutch 4, the torque-dependent pressure in the contact pressure cylinders for the conical disk pairs of the variator 12, the pressure in the adjusting cylinders of the conical disk pairs of variator 12 for altering its transmission ratio, and the control clutches K1 and K2. Not shown are gear sets or a clutch and/or a brake for reverse travel.
The structure and function of the individual components described are known and will therefore not be explained in detail.
FIG. 2 shows an example of a power-branched transmission with a variator 12, whose one conical disk pair 30 is non-rotatably connected with the input shaft 6 and can be coupled with a first gear 32 through a first control clutch K1.
The other conical disk pair 34 of the variator 12 is non-rotatably connected with an output shaft 36, which, in turn, is non-rotatably connected with the sun gear 37 of a gear transmission 14 in the form of a planetary transmission. The output shaft 36 can further be coupled through a control clutch K2 with a second gear 38 that is in rotary engagement with the first gear 32 through an idler gear 40. The second gear 38 is non-rotatably connected with the planet carrier 42 of the planetary transmission, whose planet gears 44 mesh with the ring gear 46 that is non-rotatably connected with the output shaft 10. When control clutch K2 is engaged and control clutch K1 is disengaged, the sun gear 37 and the planet carrier 42 rotate together so that the planet gears 44 stand still and drive the ring gear 46. The entire power-branched transmission then operates like a simple CVT transmission whose overall ratio is utilized doubled in accordance with FIG. 3. In FIG. 3, the abscissa represents the transmission ratio ivar of the variator and the ordinate the transmission ratio iges of the entire power-branched transmission.
Proceeding from the greatest possible transmission ratio (starting transmission ratio; from the upper right in FIG. 3), the overall transmission ratio declines linearly along the low branch with increasing transmission ratio of the variator until the shift point U is reached, at which the transmission ratio ivar has a low, predetermined value. The control clutches K1 and K2 are shifted at the shift point U so that the planet carrier 42 henceforth rotates with the given transmission ratio in accordance with the transmission ratio between the first gear 32 non-rotatably connected with the input shaft 6, the idler gear 40 and the second gear 38 in accordance with the input shaft 6, and the planetary transmission 14 becomes operative. The transmission ratios are selected in such a way that the overall ratio iges of the power-branched transmission is independent of the shift condition of the control clutches K1 and K2 at the shift point U. If the spread range of the variator 12 is traversed again, then the transmission ratio iges changes along the high branch (high speed region) represented in FIG. 3. The R branch reproduces the transmission ratio conditions for the reverse travel region. It is apparent that other curve gradients are also possible as a function of the construction of the power-branched transmission.
FIG. 4 shows another example of a power-branched transmission, in which the transmission ratios are as shown in FIG. 5, according to the actuation position of the control clutches K1 and K2. Such transmissions are designated as “geared neutral” transmissions since with the transmission ratio ivar=G of the variator, theoretically a positively or negatively infinite transmission ratio results when the transmission is situated in the low branch condition.
FIGS. 6 and 7 show a further example of a power-branched transmission in which, in accordance with FIG. 6, the variator transfers torque from the lower disk set to the upper disk set when the control clutches K1 and K1′ are engaged and the control clutches K2 and K2′ are disengaged, whereas it transfers torque downward from above when control clutches K1 and K1′ are disengaged and control clutches K2 and K2′ are engaged. The torque transmission direction consequently reverses itself at the shift point. That results in the transmission ratio conditions in accordance with FIG. 7.
FIG. 8 shows the basic structure of a further power-branched automatic transmission with a variator 12 and two planetary transmissions 14 and 14′.
The overall transmission ratio iges of the transmission in accordance with FIG. 8 as a function of the transmission ratio ivar of the variator is represented in FIG. 9.
With CVT transmissions, a common method for establishing the transmission ratio consists in carrying out a rotational speed regulation that sets a target engine rotational speed while the transmission ratio of the transmission is changed in such a way that the target engine rotational speed is set. The target engine rotational speed is determined by evaluating a characteristic curve as a function of the actuation of an accelerator pedal. In any case, an actuation control system, whether as a transmission ratio regulator or as a rotational speed regulator, is necessary because of the behavior of a CVT transmission.
An obvious strategy for carrying out a range change at the shift points U, thus the predetermined shift transmission ratios, results from the transmission ratio diagrams of FIGS. 3, 5, 7, 9 in connection with power-branched transmissions. Adjustment thereby takes place in a transmission ratio range up to the shift transmission ratio, it is then shifted, and the transmission ratio is subsequently further adjusted in the new transmission ratio range. The adjustment of the variator thereby changes its direction, so that an actuation of the variator must also take place in association with the shift of the control clutches K1 and K2 (which can also be realized as brakes).
Various problems arise from the combination of several individual actuations (clutches and/or brakes, adjustment regulators) that act upon the same power train. The operation of a first actuation influences the necessity of another actuation or its operation over the power train. The following can be identified as problems for the present power-branched automatic transmission with a variator: power train oscillations that arise during shifting, or excessive wear of the frictionally-engaged elements in the variator or the endless torque-transmitting means.
The invention is based upon the object of producing a method for regulating the transmission ratio of a power-branched automatic transmission with a variator and at least one gear transmission, which makes possible comfortable shifting and does not wear the frictionally-engaged elements in the variator. The invention furthermore is based on the object of producing a power-branched automatic transmission for carrying out the method of the invention.
Accomplishing the object of the invention regarding the method is achieved with the methods described hereinafter.